1. Technical Field
This disclosure relates to circuit breakers, and more particularly, to a circuit breaker with electronic sensing and de-latching.
2. Description of the Related Art
Residential circuit breakers have historically been designed with a bimetal and magnetic yoke assembly to mechanically detect when an overload or instantaneous condition exists. When either condition exists, an armature is rotated by bending of the bimetal and therefore de-latches or trips the mechanism, thus opening a circuit.
Typical residential circuit breakers include mechanical thermal and magnetic components that provide overload and instantaneous trip functions that protect circuits. Insulated molded housings are used to enclose and separate the mechanism poles from the electrical components. Mechanical tripping is used to trip the mechanism pole by rotating an armature connected to the overload and instantaneous systems. The armature is integrated into the design to provide de-latching and re-latching functions of the mechanism. The overall breaker size is standard so that the breaker plugs or bolts into two adjacent positions of a load center or panel board.
When an overload condition exists, a bimetal will deflect due to the increased temperature. This deflection in turn rotates an armature with a latching feature generating a latch bite that interfaces with a cradle. As the armature rotates, the latch bite decreases. Once the latch bite has decreased significantly, the cradle will slide past the armature and open the circuit.
In an instantaneous event, the breaker sees a surge in current. In turn, a magnetic field is generated in the current path bimetal. The yoke and armature use the magnetic forces generated to de-latch the breaker. This magnetic field will in turn pull the armature toward the yoke. As the armature rotates toward the yoke, the latch bite is decreased until the latch bite is small enough to allow the cradle to slide past and open the circuit.
The molded housings for a single pole circuit breaker basically include two split-half molded housings for one thermal/magnetic mechanism. The molded housing includes a single open compartment which houses all of the components. For example, the bottom of the open compartment is for the trip mechanism while an upper portion of the open compartment is for electrical components. When the mechanism pole is assembled, the open compartment is closed to connect electrical components attached to the mechanism pole. The molded housings for a two pole circuit breaker are basically two molded housings for each thermal/magnetic mechanism. Each mechanism would have a bimetal, yoke, and armature assembly. Either pole could trip the mechanism and in turn trip the adjacent pole by a rotating trip bar integrated into the design. The molded housing includes an open compartment. The bottom open compartment is for the mechanism while the upper open compartment is for electrical components. A center compartment houses components needed to provide the tripping functions. When the mechanism poles are assembled, the two mechanism compartments are assembled to each side of the center compartment.
Typically, a residential circuit breaker uses a mechanical overload and instantaneous protection mechanism that requires a bimetal, yoke, and armature assembly. The assembly process requires special attention to the amount of heat applied to the bimetal during assembly. In addition, time is required to thermally calibrate each circuit breaker.
The issues related to this assembly methodology include the following. A bimetal assembly process uses multiple brazing processes during the assembly. One braze operation is needed to assemble the yoke to the bimetal. A second brazing operation is needed to braze the bimetal to a load terminal, and a third brazing operation is needed to braze a conductive braid to the bimetal. Each of the three brazing operations can damage the bimetal's multi layer material. This also could result in inconsistencies in the final product. This design type is typically known as a directly heated bimetal since a current patch is brazed to the bimetal.
During calibration, an adjustment screw is used to reposition the bimetal and thermally calibrate the circuit breaker. This adjustment effects not only the latch engagement from breaker to breaker, but also the instantaneous trip times. The disadvantages with this type of assembly method and thermal calibration process include: the amount of time needed to fabricate the device, the uncertainty in producing thermal trip times that may be inconsistent between manufacturing plant and testing facility, and the potential damage due to multiple brazing steps.